The melt extrusion of high molecular weight polyethylene into shaped structures such as tubing, pipe, wire coating or film is accomplished by well-known procedures wherein a rotating screw pushes a viscous polymer melt through an extruder barrel into a die in which the polymer is shaped to the desired form and is then subsequently cooled and solidified into a product having the general shape of the die.
In order to achieve low production costs, it is desirable to extrude the polymer at rapid rates. Higher extrusion rates may be readily obtained by increasing the rate of revolution of the extruder screw. However, this technique is subject to limitations imposed by the viscoelastic properties of the polymer substrate. Thus, at very high extrusion rates an unacceptable amount of thermal decomposition of the polymer can result. Further, extrudates having a rough surface are often obtained which can lead to formation of an undesirable pattern on the surface of the extrudate.
In Blatz, U.S. Pat. No. 3,125,547, it is disclosed that the use of 0.01–2.0 wt. % of a fluorocarbon polymer that is in a fluid state at the process temperature, such as a fluoroelastomer, will reduce die pressure in extrusions of non-fluorinated polymers such as high and low density polyethylenes and other polyolefins. Further, use of this additive allows significant increase in extrusion rates without melt fracture.
More recently, improved fluoropolymer process aid compositions have been disclosed in, for example, U.S. Pat. Nos. 4,855,360; 4,904,735; 5,106,911; 5,587,429; 5,707,569; 6,242,548 B1; 6,277,919 B1 and 6,642,310 B2.
Linear low density polyethylene (LLDPE) resins that were manufactured in a process employing a metallocene catalyst (referred to hereinafter as mLL resins) have particularly poor rheology for blown film production, i.e. low melt strength (low elongational viscosity) and relatively little tendency to shear thin. Low melt strength generally results in soft and unstable blown film bubbles, leading to gauge variations. Lack of shear thinning results in high extrusion power requirements as well as high barrel and die pressures. To alleviate these problems, film processors often find it useful to blend low density polyethylene (LDPE) with mLL resins, because LDPE resins display excellent melt strength and shear thinning characteristics. LDPE resins are made via a high pressure, free-radical polymerization process. Although the physical properties of LDPE are far inferior to those of mLL resins, blends containing less than 50% LDPE have physical properties closer to those of mLL resins than to those of LDPE resins, while having much improved processability.
Additionally, mLL resins often have a low critical shear rate for the onset of melt fracture and thus readily exhibit melt fracture. Surprisingly, mLL-LDPE blends can display a form of melt fracture that is resistant to elimination by most fluoropolymer process aids. For example, when a mLL-LDPE blend is extruded at 500 1/s shear rate, the resulting film is fully melt fractured. Introducing a fluoropolymer process aid typically improves the film surface smoothness considerably, but defects taking the shape of ellipses may remain in the areas where the process aid has taken effect. As a result, once the fluoropolymer process aid has done the job of clearing the “hard” fracture, the film can still be covered with unacceptable defects. In mild cases, the ellipses are very light and infrequent, and the film may be acceptable. In other cases, the ellipses are so numerous and large that they merge together to form a continuous fracture pattern. Interestingly, increasing fluoropolymer process aid level in the mLL-LDPE blend only makes the ellipse defects worse, so that film processors often cannot find a window in which to operate between the extremes of low process aid level, where hard fracture may return, and high process aid level, which aggravates the ellipses.
Thus there is a need for a process aid composition which reduces melt fracture in mLL-LDPE blends without introducing elliptically shaped defects in the resulting films.